1. Field of the Invention
The present invention relates to an image processing apparatus, an image forming method, and a computer product.
2. Description of the Related Art
Japanese Patent Application Laid-Open No. 2002-325179 discloses a technology such that an invention has a plurality of γ-conversion tables used to convert density, and that at least one of the γ-conversion tables has characteristics not to cause data, in a low density area that is wider than the other γ-conversion tables, to be reproduced. In the technology, the γ-conversion tables are switched by selecting a table according to information obtained from a feature extraction unit that expresses a feature of image data using multivalue, and also according to threshold values.
A technology for color copiers as follows is also known. The color copier determines whether image data for red, green, and blue (RGB image data) obtained from a scanner is any image of text, halftone dots, photograph, etc. according to a feature amount of the image (first feature amount). The color copier performs first gradation conversion on pixels that are determined as text using a gradation conversion table for text image. The color copier also performs first gradation conversion on pixels that are determined as photograph or halftone dots using a gradation conversion table for a photograph image. In this case, for example, the gradation conversion table for the text image is used for a process where reproduction of text is more preferential to gradation, and has characteristics of high-contrast gradation. The gradation conversion table for the photograph image is used for a process where gradation is more preferential to the reproduction of the text, and has characteristics of low-contrast gradation.
For improvement of image quality, there is another case. In this case, a second feature amount is extracted from image data for yellow, magenta, cyan, and black (YMCK image data), it is determined whether the second feature amount indicates text, halftone dots, or photograph, second gradation conversion is performed thereon according to each of the cases, and error diffusion is performed. For example, fixed threshold values for the error diffusion are used for pixels determined as text from the second feature amount, and quantization threshold values that periodically oscillate are used for image data determined as an area other than the text from the second feature amount, such as photograph and halftone dots. Consequently, it is possible to perform various gradation processes in which reproduction of text, reproduction of halftone dots, and gradation reproduction of a photograph image are preferentially performed, respectively.
In the technology, characteristics of a first gradation converter are switched according to text image, photograph, or halftone dots based on the result of extracting the first feature amount, and the first gradation conversion is performed. Further, it is determined whether data is for text, halftone dots, and photograph from the result of extracting the second feature amount, and then the second gradation conversion is performed. Furthermore, an amplitude of oscillations of the quantization threshold values is switched to perform the process of error diffusion.
However, under the processes, in the photograph mode, toner in the text portion flies off to cause a blurred character to occur, and therefore, there may come up such a defect on the image that a sharp character is not obtained. This is because the character is affected by changes of the gradation conversion tables (printer γ tables) during the first gradation conversion in the process of extracting feature amounts.